The present invention relates generally to ink jet printers and is particularly directed to a TAB circuit of the type which carries electrical signals to an array of nozzles on a heater chip. The invention is specifically disclosed as a TAB circuit that eliminates bent or broken electrical circuit traces before being bonded to the heater chip, while also providing a window for the nozzle plate.
TAB (Tape Automated Bonding) circuit technology has been used as the primary interconnect device between the heater chip of ink jet printers and the printer main body for many years. Conventional TAB circuits are comprised of a substrate material, usually polyimide, with some form of metallization on the substrate that forms electrical circuits. There are many patents that disclose the use of TAB circuits with ink jet printers, and most of these patents are owned by Hewlett-Packard Company of Palo Alto, Calif.
One example patent owed by Hewlett-Packard is U.S. Pat. No. 5,300,959, by McClelland. McClelland discloses a nozzle member for an ink jet printer cartridge that uses a flexible polymer tape (i.e., the TAB circuit) and affixes that tape to a substrate that contains the heating elements that create the droplets that jet forth from the nozzles. Electrical conductors that provide pathways for electrical signals to the substrate are located on the flexible polymer tape and, through an opening (or xe2x80x9cviaxe2x80x9d), make a connection to the substrate xe2x80x9celectrodes.xe2x80x9d The vias are on the back side of the tape and face the conductive portions (i.e., electrodes) of the substrate.
McClelland""s FIG. 8 illustrates a partial cross-section of portions of the TAB circuit where it interfaces to the substrate. FIG. 1 herein discloses a cross-section of a similar area of the TAB circuit interface to the substrate, but from a different angle. In FIG. 1 herein, the TAB circuit is generally designated by the reference numeral 10. The polyimide layer is designated by the reference numeral 12, and is made of a material manufactured by DuPont that is also known as KAPTON(copyright). A metal pathway or xe2x80x9ctracexe2x80x9d is provided from the left-hand side of FIG. 1 at the reference numeral 14. A similar electrical pathway or trace arrives from the right-hand side of FIG. 1 at the reference numeral 16. On the opposite side of the metal traces 14 and 16 is another layer of material at 18. This layer of material 18 is either a covercoat material, or another layer of the polyimide or KAPTON material, and which insulates and covers the metal traces 14 and 16 in the direction that faces the substrate.
The substrate itself is designated by reference numeral 20 on FIG. 1, and includes the resistive heating elements and xe2x80x9celectrodesxe2x80x9d that make up what is commonly known as a xe2x80x9cheater chip.xe2x80x9d Two of the electrical pathways or electrodes are at the reference numerals 22 and 24, and two of the resistive heating elements are at the reference numerals 26 and 28. For example, the electrical signal that arrives at the electrode 22 could travel through a metal trace or pathway (not shown on FIG. 1) to connect to the heating resistor 26, and when energized by a sufficient electrical power level, the heating resistor 26 will cause a droplet of ink to be spurted out through a nozzle opening at 34 in the TAB circuit 10. Continuing this example, the electrode 24 could be connected using metal pathways or traces to the other heating resistive element on FIG. 1 at 28. When a sufficiently powerful electrical signal arrives at the heating resistor 28, this will cause a droplet of ink to be spurted out through a nozzle opening 36 in the TAB circuit 10.
The electrical connection between the metal trace 14 and the xe2x80x9celectrodexe2x80x9d 22 is created by a via or opening 30 in the covercoat layer 18 of the TAB circuit 10. This via must be filled with some type of electrically conductive substance, which could be a conductively filled polymer. Or perhaps a reflow soldering method could be used, or even an ultrasonic welding procedure. In a similar manner, an electrical connection can be made between the trace 16 and the xe2x80x9celectrodexe2x80x9d 24 through the via 32 in the covercoat layer 18 of the TAB circuit 10.
In the McClelland patent as illustrated in FIG. 1 herein, there is no separate nozzle plate (or xe2x80x9corifice platexe2x80x9d) that forms the nozzle openings of most conventional ink jet cartridges. Instead, the TAB circuit 10 itself covers the entire nozzle area, including the middle area 38 between the nozzles 34 and 36.
For example, U.S. Pat. No. 5,278,584 (by Keefe) discloses an ink jet printhead that has improved ink flow paths between the ink reservoir and vaporization chambers. FIGS. 3 and 4 of Keefe illustrate the structure of the electrically conductive leads that are attached to the substrate. These conductive leads or traces are initially unsupported before being bonded to electrodes on the substrate. One advantage of the Keefe design is that the electrical traces that bring signals to the electrodes on the heater chip can all be temporarily run to a shorting bar (not shown) that can provide a single common electrode for an electroplating process for all of the circuit pathways of the TAB circuit itself. The McClelland design may not lend itself well for creating such a shorting bar. In Keefe, the shorting bar can be removed along with a portion of the polyimide material that creates a xe2x80x9cchip windowxe2x80x9d in the nozzle area, and by which a nozzle plate can be installed through that chip window in the TAB circuit.
Conventional ink jet cartridges manufactured by Lexmark International, Inc. of Lexington, Ky. use a similar design to that disclosed in Keefe. One example of such similarity is the fact that the metal traces which carry electrical signals to the heater chip are initially unsupported at their terminal ends before a bonding procedure can be performed between the TAB circuit and the heater chip. Moreover, a xe2x80x9cchip windowxe2x80x9d in the TAB circuit is created for installation of a nozzle plate, and also for the removal of the temporary shorting bar that provides an easy-to-access point used during the electroplating process of the metal traces of the TAB circuit itself. These unsupported circuit traces (also known as unsupported lead beams) extend into the chip window opening in the polyimide of the TAB circuit, and are later thermosonically bonded to metal contact pads on the heater chip. This chip window is formed by creating an internal edge through the polyimide that defines a closed perimeter, thereby forming a boundary (i.e., the internal edge).
FIG. 2 herein illustrates a portion of a TAB circuit used in a conventional Lexmark ink jet cartridge in the nozzle area. The polyimide material is generally designated by the reference numeral 50, and is cut or otherwise etched along an edge at 52 that creates an opening or chip window 74. This edge 52 will also be referred to herein as a xe2x80x9cPI edge.xe2x80x9d As part of the artwork that makes up this TAB circuit, a relatively large plus sign (xe2x80x9c+xe2x80x9d) is provided at 54 to aid in registration when mating the TAB circuit to the heater chip.
On FIG. 2, four different metalized circuit pathways or traces are illustrated at 60, 61, 62, and 63. These metal traces 60-63 each have an end point, designated respectively at the reference numerals 65, 66, 67, and 68. As can be easily seen on FIG. 2, these traces at their end points 65-68 terminate along a different line or plane than the PI edge 52. As also can easily be seen in FIG. 2, these traces 60-63 are initially unsupported, as they extend past the PI edge 52 into open space.
Also as part of the TAB circuit on FIG. 2 is a covercoat layer that is not visible in the figure, since it is on the opposite side of the TAB circuit. However, the edge of this covercoat layer is indicated by a hidden line at the reference numeral 70 on FIG. 2. This covercoat material extends over the metal traces, which affects the shape of the edge 70 as indicated at 72, as the edge 70 follows over the outline of the metal trace 61.
FIG. 3 shows a further step in the process of constructing an ink jet cartridge known in the prior art. This further step now includes the heater chip at 85, and also includes a nozzle plate at 95. The heater chip includes multiple bonding pads, such as those indicated by the reference numerals 80, 81, 82, and 83. On FIG. 3, these bonding pads 80-83 mate respectively to the unsupported lead beam ends of the traces 60-63. So long as the traces 60-63 have not been bent, the ends of the traces 65-68 will optimally match up to the physical locations of the bonding pads 80-83 when overall TAB circuit registration is correct with respect to the heater chip 85.
The nozzle plate 95 includes multiple openings or orifices, which are designated at the reference numerals 90, 91, 92, and 93. These nozzle openings 90-93 generally match up to heater elements on the heater chip 85, and these heater elements (not shown on FIG. 3) will generally match up to the electrical signals that are brought to the heater chip 85 by the metal traces 60-63. One advantage of using a separate nozzle plate 95 is that its registration with respect to the heater chip 85 is not dependent upon the registration of the TAB circuit 50 with respect to the same heater chip 85. Therefore, more accurate placement of the nozzle openings 90-93 can be made by the independent nozzle plate 95 (irrespective of the TAB circuit 50).
As noted above, the lead beams 60-63 extend into an opening in the polyimide of the TAB circuit, and are unsupported when they are shipped from the TAB circuit manufacturer. The delicate nature of the lead beams often results in bent leads in all three directions. Bent leads can occur during the actual circuit manufacturing, or later in the TAB circuit-to-heater chip attachment process. The result is higher scrap rates at the TAB circuit manufacturer, and lower yields and extra scrap at the ink jet cartridge manufacturing process. Of course, all of this scrap increases the cost of the TAB circuit and the ink jet cartridge manufacturing process.
Another shortcoming of the conventional manufacturing procedures and conventional design illustrated in FIGS. 2 and 3 is the location of the physical interface between the metal traces, the PI edge, and the covercoat material. One purpose of the covercoat material is to prevent the corrosive ink from contacting the metal traces. Due to the covercoat placement tolerances, however, there are occurrences when the ink reaches the metal traces at the interface, thereby leading to lead beam corrosion (of these traces). This does not automatically occur, but depending upon tolerances, there could be such an opportunity for corrosion. Lead beam corrosion can cause ink jet cartridge failure either marginally or catastrophically, and will potentially render the cartridge unreliable.
A typical method of fastening the TAB circuit 50 to the heater chip 85 is the use of thermosonic welding. Traditionally the unsupported lead beams of the TAB circuit are aligned to the individual bond pads of the heater chip so as to perform the assembly process. Since the lead beams 60-63 are unsupported and are independent members, a Video Lead Locator (VLL) inspection process is required to visually locate each lead beam so that all bent leads are identified and skipped prior to the thermosonic welding procedure. TAB circuits with bent leads are deemed unusable in the TAB bonding process and are scrapped. The VLL inspection process adds cycle time per circuit, and in some assembly equipment it requires more than three seconds of additional cycle time per circuit.
The traditional TAB bonding process inherently must accept the variation in the location of the unsupported lead beams. The fact that the traditional TAB circuit has potential variation in the exact x, y, and z locations of the independent lead beams creates additional variability in the TAB bonding process, and ultimately affects the quality of the thermosonic welds. The quality of the thermosonic welds is typically measured by destructive pull force testing, or by destructive shear testing methods at the weld joint. An off-location lead beam typically has a reduced weld strength and typically produces a lower pull force or shear force to separate the lead beam from the bond pad on the heater chip.
It would be an improvement to provide a TAB circuit that eliminated the unsupported lead beams that bring the electrical signals to the heater chip, and also that would tend to eliminate the possibility of corrosive ink coming into contact with the metal circuit traces (i.e., the lead beams) that carry these important signals to the heater chip.
Accordingly, it is an advantage of the present invention to provide a TAB circuit that provides a polyimide edge to create a chip window for a nozzle plate, in which the metal circuit traces are not unsupported at their ends where they make contact with bond pads of a heater chip. It is another advantage of the present invention to provide TAB circuit that provides an improved tolerance for a covercoat placement that will tend to prevent corrosive ink from coming into contact with the metal traces that carry the electrical signals to the heating elements of the heater chip.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, a tape automated bonding circuit for a printer cartridge is provided, which includes a flexible substrate having a first surface and a second surface and comprised of electrically insulating material, in which a plurality of electrically conductive traces are affixed to the first surface of the flexible substrate. The flexible substrate includes a chip window opening that is formed between the first and second surfaces by an internal edge in the flexible substrate that defines a closed perimeter. At least a subset of the plurality of electrically conductive traces forms pathways that terminate substantially adjacent to the internal edge.
In accordance with another aspect of the present invention, a method is provided for assembling a tape automated bonding circuit used in a printer cartridge, including: (a) providing a flexible substrate comprised of electrically insulating material, and a plurality of electrically conductive traces that are affixed to a first surface of the flexible substrate. The flexible substrate includes a chip window opening that is formed in the first surface by an internal edge in the flexible substrate that defines a closed perimeter, and at least a subset of the plurality of electrically conductive traces forms pathways that terminate substantially adjacent to the internal edge; (b) providing a heater chip that exhibits a plurality of bond pads along at least one edge of the heater chip, in which the heater chip contains a plurality of resistive heating elements. An inner dimension formed by at least one of a length or a width of the chip window is smaller in linear distance than an outer dimension formed by at least one of a length or a width of a surface of the heater chip; (c) aligning the heater chip and the flexible substrate-using at least one registration mark on the flexible substrate but without the use of a video lead locator inspection tool to detect bent leads-so that the end of the pathways of the at least a subset of the plurality of electrically conductive traces comes into close proximity to the plurality of bond pads along the at least one edge of the heater chip, such that a portion of the flexible substrate overlaps a portion of the surface of the heater chip along the at least one edge of the heater chip; and (d) bonding the plurality of bond pads to corresponding of the at least a subset of the plurality of electrically conductive traces.
In accordance with a further aspect of the present invention, a method is provided for partially assembling a tape automated bonding circuit used in a printer cartridge, including: (a) providing a flexible substrate comprised of electrically insulating material, and a plurality of electrically conductive traces that are affixed to a first surface of the flexible substrate. The flexible substrate includes a chip window opening that is formed in the first surface by an internal edge in the flexible substrate that defines a closed perimeter, and at least a subset of the plurality of electrically conductive traces forms pathways that terminate substantially adjacent to the internal edge; (b) providing a heater chip that exhibits a plurality of bond pads along at least one edge of the heater chip, in which the heater chip contains a plurality of resistive heating elements. An inner dimension formed by at least one of a length or a width of the chip window is smaller in linear distance than an outer dimension formed by at least one of a length or a width of a surface of the heater chip; (c) aligning the heater chip and the flexible substratexe2x80x94using at least one registration mark on the flexible substrate but without the use of a video lead locator inspection tool to detect bent leadsxe2x80x94so that the end of the pathways of the at least a subset of the plurality of electrically conductive traces comes into close proximity to the plurality of bond pads along the at least one edge of the heater chip, such that a portion of the flexible substrate overlaps a portion of the surface of the heater chip along the at least one edge of the heater chip; and (d) applying an adhesive material between the flexible substrate and the heater chip at locations in a support web area of the flexible substrate where it overlaps at least a portion of the surface of the heater chip along the at least one edge of the heater chip, thereby tacking the flexible substrate to the heater chip in an aligned position to create a sub-assembly that can be permanently electrically connected in a subsequent procedure.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.